When the document uses a font that cannot be found on the machine, it is substituted for another font. What should happen is to fall back to another font for the visualization, but to preserve the original font in the model.

Partial workaround: for HTML import, the font can be preserved by subscribing to the FontSubstituting event:
provider.ImportSettings.FontSubstituting += ImportSettings_FontSubstituting;
private void ImportSettings_FontSubstituting(object sender, FontSubstitutingEventArgs e)
{
      e.SubstitutionFontFamily = new FontFamily(e.OriginalFontName);
}